linux-kernel - Re: [PATCH 2/2] tools/memory-model: Add write ordering by release-acquire and by locks

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Message-Id: <20180705233257.GY3593@linux.vnet.ibm.com>
Date:   Thu, 5 Jul 2018 16:32:57 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     Daniel Lustig <dlustig@...dia.com>,
        Will Deacon <will.deacon@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] tools/memory-model: Add write ordering by
 release-acquire and by locks

On Thu, Jul 05, 2018 at 08:44:10PM +0200, Andrea Parri wrote:
> On Thu, Jul 05, 2018 at 08:38:36PM +0200, Andrea Parri wrote:
> > > No, I'm definitely not pushing for anything stronger.  I'm still just
> > > wondering if the name "RCsc" is right for what you described.  For
> > > example, Andrea just said this in a parallel email:
> > > 
> > > > "RCsc" as ordering everything except for W -> R, without the [extra]
> > > > barriers
> > 
> > And I already regret it: the point is, different communities/people have
> > different things in mind when they use terms such as "RCsc" or "ordering"
> > and different communities seems to be represented in LKMM.
> > 
> > Really, I don't think that this is simply a matter of naming (personally,
> > I'd be OK with "foo" or whather you suggested below! ;-)). My suggestion
> > would be: "get in there!! ;-) please let's refrain from using terms such
> > as these (_overly_ overloaded) "RCsc" and "order" when talking about MCM
> > let's rather talk, say, about "ppo", "cumul-fence" ...
> 
> ... or bare litmus tests!

Validation of changes to the memory model is going to continue to be
an interesting topic, which will probably involve its share of litmus
tests.

							Thanx, Paul

>   Andrea
> 
> 
> > 
> >   Andrea
> > 
> > 
> > > 
> > > If it's "RCsc with exceptions", doesn't it make sense to find a
> > > different name, rather than simply overloading the term "RCsc" with
> > > a subtly different meaning, and hoping nobody gets confused?
> > > 
> > > I suppose on x86 and ARM you'd happen to get "true RCsc" anyway, just
> > > due to the way things are currently mapped: LOCKed RMWs and "true RCsc"
> > > instructions, respectively.  But on Power and RISC-V, it would really
> > > be more "RCsc with a W->R exception", right?
> > > 
> > > In fact, the more I think about it, this doesn't seem to be RCsc at all.
> > > It seems closer to "RCpc plus extra PC ordering between critical
> > > sections".  No?
> > > 
> > > The synchronization accesses themselves aren't sequentially consistent
> > > with respect to each other under the Power or RISC-V mappings, unless
> > > there's a hwsync in there somewhere that I missed?  Or a rule
> > > preventing stw from forwarding to lwarx?  Or some other higher-order
> > > effect preventing it from being observed anyway?
> > > 
> > > So that's all I'm suggesting here.  If you all buy that, maybe "RCpccs"
> > > for "RCpc with processor consistent critical section ordering"?
> > > I don't have a strong opinion on the name itself; I just want to find
> > > a name that's less ambiguous or overloaded.
> > > 
> > > Dan
>